Not all Ultrasound Therapy Devices perform the same. A High Efficient Ultrasound Treatment Head may be more expensive than the price of a whole device of another make. The difference is the efficiency of the ultrasound production.

When a piezo-crystal is charged by electrical signal, the crystal shrinks and expand and this vibration action of piezo-crystal emits sound wave.

In therapeutic ultrasound, the intensity/dosage is quoted as "Watt/cm²" which is also known as SPATIAL AVERAGE INTENSITY (SAI), the ultrasound control module is programmed to adjust electrical charge according to the EFFECTIVE RADIATING AREA (ERA) of the piezo-crystal of a particular ultrasound treatment head in order to deliver the SAI set by a therapist. The tricky part is that every piece of piezo-crystal performs slightly different therefore the ERA of two piezo-crystal are not exactly the same; even from same model of device from same manufacturer. According to independent study, SAI error in some devices can be as much as 53%. Few reliable manufacturers will measure the ERA of each of their ultrasound heads and then fine tune the ultrasound control software accordingly; while other manufacturers commonly use a fixed ERA value to program their ultrasound control software which will result inaccurate production of SAI. Also, whether the electrical signals can be efficiently converted into mechanical signals, i.e. sound wave, depends on the quality of piezo-crystal and engineering skill. These factors explain why endusers have reported some ultrasound heads seemed more powerful than others at same intensity setting.

Another obvious sign of inefficient ultrasound production is that the front part of ultrasound head (where the sound wave is emitted) will become hot. If electrical signals cannot be converted into mechanical signals completely, the excessive energy would turn into heat; which is simple law of physics. Ultrasound treatment is capable of increasing the temperature of the tissue inside a treatment area when thermal therapy goal is required but not heating up the ultrasound head itself.

Some makes claim their ultrasound heads offer pre-warm feature, unless a manufacturer provides proof that additional electronic circuit is built in the ultrasound head to produce warming effect, the thermal reaction on the ultrasound head is probably the by-product of inefficient ultrasound production which means a lower output intensity and a shorter life of the piezo-crystal.